This invention concerns a grinding machine of the type used during the manufacture of flat, board-shaped workpieces, especially for boards made from particle-board materials, fiber materials, veneer materials, and mineral materials, for example materials containing calcium silicate or other similar boards, with at least one continuous belt unit, consisting of a grinding belt that can be moved or guided over rollers by means of at least one contact element, against the surfaces being ground, and of one counterpressure unit, according to which the workgap between the contact element and the counterpressure element is adjustable in relation to the thickness of the board being ground.
Grinding machines known in the prior art, which display at least a continuous belt unit of the kind just described, make it possible to grind a surface at a certain treatment depth, or to deposit a surface detail in such a manner that, depending on the case, the upper and lower side of the board may be treated in the same or different ways. This is often important when, because of one or another reasons determined by the board manufacturing process, different technical conditions exist on one or both board surfaces; it may be necessary, according to the exigencies for further processing, to be able to provide the boards with a surface detail or refinement by printing, painting, or sheathing in paper, leaf, veneer, or similar material (cf. Holztechnik, October 1973, p. 378-379, 388; cf. Holz als Roh-und Werkstoff, 38, 1980, p. 321; Broschure der Carborundum-Werke GmbH, Plant 2, Publication No. 72/3-8000, p. 80-88). According to the grinding process, boards are produced, in general, whose thicknesses lie preferably within prescribed tolerances. Typical allowable tolerances are .+-.0.3 mm, corresponding DIN 68 761, and in some countries, .+-.0.15 mm from the nominal thickness (Deppe, H. J.; Ernst, K., Technologie der Spanplatten, Stuttgart, 1964, pp. 191-192). In contrast, high-power continuous-belt grinding machines guarantee a thickness tolerance of a maximum of .+-.0.1 mm, related to the nominal thickness, as well as relatively high forward feeding speeds of around 30 to 40 m/min. They also provide for variable thickness differentials from board to board. In addition, this process assumes a fault-free grinding pattern in order to insure that the boards conform to the demands of further processing. (DE-PS 1977 135, DE-PS 2502 718, Assignee's Prospectus, 12.79).
If, for example, ground boards are to be stacked in a normal manner in great numbers one on top of the other before they are further treated and processed, the problem arises that the boards, especially the ones towards the top of the stack, are increasingly deformed. The degree of this deformation depends essentially not only on the height of the stack, the number of boards in it, the thickness and hardness of the boards, but rather also more especially on the overall accumulative sum of the tolerance values, or the tolerance errors, which can deviate either positively or negatively in each board from the optimum value; it also depends on the thickness of the boards, because heavier, more tightly compressed boards will be deformed to a greater extent than light boards.
By way of example, this means that, in a stack 4 meters high, which is often met in practice, 400 pieces of ground particle boards, 10 mm thick, relatively heavy and tightly packed will be present. In the upper part of the stack of the stacked boards, there may be a bending across their widths (bowing) of at least up to 40 mm when the total deviation per board is 0.2 mm (10.1 mm maximum in the middle of the board, 9.9 mm minimum at the boundary region of the board). In order to make the example more comprehensible, the tolerance errors in the boards, which may be sampled from a day's production before and after grinding, have not been taken into consideration here. If the case concerns particle board, as it does here, then the bonding agent in them hardens while they are stacked and continues to harden after they are unstacked; this means that the variations in the shape over the boards' widths, due to the setting of the boards' internal consistency, is irreversible. Boards of this kind, which, under certain circumstances, are grouped together according to their formats, can no longer be processed in furniture manufacture for this reason.
In connection with the problems outlined in the foregoing, the flawless glueing together of a large number of thin boards, with or without an accompanying sheathing material, in a stack press is precluded. It is in this respect that the accumulated tolerances of the boards cause misbonding patches.
It is the purpose of the invention to create a grinding machine of the type defined at the outset which is adaptable in such a way to the changing loading conditions during the grinding operation resulting from the variable properties (characteristics) of the boards being ground so that boards true to size, and with uniform surface qualities, may be obtained.
This purpose is realized in such a way by the invention that the counterpressure element on the side opposite the work gap is fitted with a support device at least at the midpont along its length that can be fed with a variable pressure.
The basic advantage of this invention's solution, in substantial contrast to prior and present state-of-the-art, consists in the fact that the work gap between the contact element and the counterpressure element, across its entire width, and under the just-named variable loading conditions, is maintained in a plane-parallel position (i.e., they lie parallel to each other in parallel planes); at a certain point on the counterpressure element, a supplementary and variable pressure can be brought to bear. Because this pressure can be used with structural components and subassemblies of the continuous belt unit already in existence, their compact and space-saving design principles may be retained.
By creating a specific condition of plane-parallelism in the work gap, the invention assures that, during the production of boards with variable properties (characteristics), which differ sharply from one another, they may be ground free of faults, both with respect to the quality of their surfaces, as well as with respect to precision in maintaining uniformity in their sizes. A thickness tolerance of upwards of .+-.0.05 mm, with respect to the nominal thickness, may be assured with confidence and without thickness tolerance errors. The uniformly ground boards are, in this way, even in shape and, when they are stacked one on top of another in piles, they will not be subject to variations in shape. In this way, the disadvantages that have been inherent up to the present, for instance, in stack presses, can be circumvented.
According to this invention, the supporting mechanism may be realized in various ways. Thus, a counterpressure roller, consisting of a steel cylinder with smooth lateral surface areas, may be supported by a backing roller on the side opposite the work gap along its midpoint; both its outer bearings lie over a fork bracket which is driven by a hydraulically or pneumatically impelled piston. This piston can be precisely installed to work in a discontinuous or continuous mode and in relation to the plane-parallel arrangement of the work gap between the contact element and the counterpressure roller, using conventional or modern control techniques.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.